This application claims priority to and hereby incorporates by reference Japanese Patent Application No. HEI 11-180285, which was filed on Jun. 25, 1999, and hereby incorporates by reference Japanese Patent Application No. HEI 10-336395 which was filed on Nov. 26, 1998.
1. Field of the Invention
The invention relates to a discharge lamp, and more particularly relates to a metal halide lamp that does not contain mercury; The discharge lamp is preferably incorporated in a vehicle headlamp.
2. Description of Related Art
Various types of metal halides are contained in the arc tubes of high-pressure mercury or typical metal halide lamps in order to ensure light emission in the desired spectral distribution. Metal halides are solids at room temperature. When an arc tube wall is heated by an arc discharge, solidified metal halides located at the tube wall vaporize and metal-specific light emissions are obtained.
The temperature of gas and ions within a discharge medium is dependant on the pressure of the medium. The pressure and temperature within the arc tube are therefore high in order to cause the mercury, which is of a relatively high vapor pressure, to vaporize, along with the metal halides. Related metal halide lamps therefore require both inert gases (starter gases) to start discharge, and mercury, in order to create high pressure within the tube and to increase tube wall temperature.
A starter gas is used for starting discharge and usually, argon gas is enclosed within a range of 1 kPa to 10 kPa In this pressure range, the temperature of the rare gases and ions within the discharge portion is not so different from room temperature. The temperature of the walls of the arc tube gradually rises at the start of discharge. In a comparatively short time, the vapor pressure of the mercury rises as the tube wall temperature exceeds 300xc2x0 C., and a high temperature arc (hot plasma) is generated. The tube wall temperature then rapidly rises and the metal halide vaporizes. When there is no mercury present within the lamp, the tube walls do not heat up until a temperature is reached where the evaporation pressure of the metal halogen compound occurs. Effective luminous flux is therefore not obtained in typical metal halide lamps that do not have mercury.
In recent years, metal halide lamps have begun to require remarkably low power, with 35 W arc tubes being adopted for vehicle headlamps. Vehicle headlamps are required to light-up instantaneously and therefore contain a small amount of xenon gas which is used as a starter gas. The xenon emits light when the lamp is lit, and practically instantaneous illumination can be achieved by generating a thermal plasma from the beginning of power supply so as to rapidly heat the arc tube.
With metal halide lamps for vehicle use, mercury is necessary in order to create a high Lit pressure condition inside of the arc tube and to sufficiently raise the temperature of the tube walls. However, mercury is a toxic material, and if part of the arc tube is damaged, mercury will be leaked into the surrounding environment. Mercury has, however, been widely used in metal halide lamps with no suitable replacement. When such arc tubes are disposed, it is necessary to break up the arc tubes and recover the mercury, which increases costs. In recent years, arc tubes that do not include toxic materials, such as mercury, have become preferred.
Ultraviolet rays are not required in a large number of lighting applications. However, metallic vapor discharge lamps including mercury may cause damage to the subject of illumination as a result of the emission of ultraviolet rays from the mercury. A great deal of work and cost is involved in blocking these ultraviolet rays. Further, while the arc tube is starting up, the arc tube appears tinged with blue and color rendering is poor in a period where the mercury vapor pressure is rapidly rising, which makes limits on the use of mercury unavoidable. Short arc xenon lamps are available as high-intensity discharge lamps that do not include mercury, but lamp efficiency is low at approximately 30 lumens per watt. Thus, these lamps cannot be used in applications where efficiency is important.
Additional features and advantages of the invention will be set forth in the description that follows, and in part, will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and achieved by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
The invention is directed to a discharge lamp that resolves the aforementioned problems by providing a metal halide lamp where mercury is not enclosed within the arc tube, so that ultraviolet rays are not emitted by the mercury. Thus, it is no longer necessary to block ultraviolet rays, and it is not necessary to dispose of mercury. A discharge lamp can therefore be provided that is cheaper and resolves the problems of related metal halide lamps.
FIG. 4 is a view showing spectral distribution of light emitted by the arc tube, with solid lines showing spectral distribution of light emitted by a prior mercury-free arc tube and the broken lines showing spectral distribution of light emitted by a mercury-containing arc tube. As shown in FIG. 4, with the arc tube containing a metal-halogen compound of scandium iodide and sodium iodide that does not contain mercury, the generation of light in the blue-light band of 404 nm to 435 nm etc. by the mercury no longer occurs, and the blue light wavelength component is weak and deviates out of the white light range of the chromaticity coordinates.
Light sources for vehicle use require that 25% of the rated luminous flux be generated within one second from the start of discharge, and 80% of the rated luminous flux be generated within four seconds from the start of discharge. It is difficult to achieve the flux required after four seconds in the absence of mercury.
It is the object of the invention to provide a metal halide lamp for vehicle use that does not contain mercury so as to improve the chromaticity and start characteristics.
In the invention, a discharge lamp is equipped with a pair of electrodes facing each other in a discharge space within an arc tube. A metal halide and a rare gas are enclosed in the discharge space and the rare gas is enclosed at a high pressure so as to create a hot plasma of high temperature and pressure. The heat capacity and heat loss of the arc tube are suppressed, raising of tube wall temperature is promoted, and the metal halide compound vaporizes in such a manner as to emit light. The metal halide contains at least scandium iodide or sodium iodide.
Here;
P/(Qxc2x7t)xe2x89xa70.20
where Q is the content volume of the arc tube (xcexcl), t is maximum wall thickness (mm), and P is pressure of the xenon gas at room temperature (atms).
Moreover;
P/S1/S2xe2x89xa70.06
where S1 is a cross-sectional area of a portion of the greatest internal diameter of the discharge space of the arc tube (mm2), and S2 is a cross-sectional area of material forming the portion of the greatest internal diameter of the arc tube (mm2).
A metal halide lamp with a pair of electrodes projecting in such a manner as to face each other in a discharge space within an arc tube, with mercury not being included in the discharge space, and with a substantially cylindrical arc being generated between ends of the pair of electrodes, is provided. In this metal halide lamp, a buffer gas serving as a starter gas comprising xenon of approximately 7 to 20 atms at room temperature; sodium halide, scandium halide, or a compound thereof; and a low melting point metal halide with a melting point of approximately 400xc2x0 C. or less are enclosed in the discharge space. As a result, similar light-emitting characteristics as realized in conventional metal halide lamps can be achieved without using any mercury.